Integrated circuits that include high-voltage transistor switches, for example, field-effect transistor (HVFET) structures are commonly utilized for a multitude of purposes and applications. In some instances an HVFET is used in a circuit for discharging an electromagnetic interference (EMI) filter capacitor, referred to as an x-capacitor. The x-capacitor is typically coupled directly across the input terminals of a power converter circuit. In one example, a discharge circuit including one or more HVFETs may be coupled across an x-capacitor at the input terminals of a power converter. The discharge circuit may detect whether an ac electrical energy source is coupled to the input terminals of the power converter. If the ac electrical energy source is disconnected, the timing and control circuit drives the HVFET(s) on, thereby providing a path to ground such that the x-capacitor may discharge quickly.
In another example, a discharge load (i.e., one or more series resistors) is coupled across the capacitor to discharge the charge on the x-capacitor and thus drops the voltage across the input terminals within the specified time. However, using a discharge load results in substantial losses due to power dissipation when the power supply is connected to the ac electrical energy source during operation. Since power dissipation with a discharge load is substantially constant and independent of load conditions, during no/light load conditions the power dissipation may be a larger percentage of the output power, which may reduce power converter efficiency during light/no load operation; however, efficiency may be improved at higher load levels.
The use of an active switching element coupled in series with the dissipative resistors is one solution taught in the '457 patent. When the switching element is off, power is not being dissipated in the discharge load, but when the ac line voltage is removed and the x-capacitor needs to be discharged, the switching element turns on and connects the discharge load across the x-capacitor, thus discharging the x-capacitor within the specified time. One critical aspect that arises in the use of an active switching element is promptly and correctly determining when the input terminals have been decoupled or disconnected from the ac input line voltage.